There has been a conventionally well-known method for obtaining a large-screen image, in which an optical image corresponding to a video signal is formed on a light valve, irradiated with light, and then magnified and projected onto a screen by a projection lens. The use of a reflecting light valve as the light valve can achieve both a high resolution and a high pixel aperture ratio, making it possible to display a highly-bright projected image with highly efficient light utilization.
As a conventional example, FIG. 10 illustrates a configuration of an optical system of a projection-type display apparatus using a reflecting light valve. An illuminating optical system for focusing and illuminating light emitted from a lamp 1 as a light source onto a reflecting light valve 6 includes a concave mirror 2, a quadratic rod prism 3 whose cross-section has substantially the same aspect ratio as an effective display surface of the reflecting light valve 6, a condenser lens 4 and a focusing mirror 5.
The concave mirror 2 has a reflecting surface with an elliptical cross-section and has a first focus and a second focus. The lamp 1 is disposed so that the center of its light-emitting body is in the vicinity of the first focus of the concave mirror 2, and the rod prism 3 is disposed so that its light entrance surface is in the vicinity of the second focus of the concave mirror 2. In addition, the concave mirror 2 is obtained by forming on an inner surface of a glass base an optical multilayer film that transmits infrared light and reflects visible light.
Light emitted from the lamp 1 is reflected and focused by the concave mirror 2 and forms an image of the light-emitting body of the lamp 1 at the second focus of the concave mirror 2. The light-emitting body image of the lamp 1 is brightest near the center, which is close to an optical axis, and rapidly becomes darker toward the periphery, and thus there is ununiformity in brightness. To solve this problem, the entrance surface of the rod prism 3 is disposed in the vicinity of the second focus, incident light is subjected to multiple reflections on a side surface of the rod prism 3 to achieve brightness uniformity, and the exit surface of the rod prism 3 serves as a secondary surface light source to form an image on the reflecting light valve 6 by the subsequent condenser lens 4 and focusing mirror 5, thereby securing uniformity of the illuminating light.
Herein, the operation of the reflecting light valve 6 will be described referring to FIG. 11. The reflecting light valve 6 controls a traveling direction of light according to a video signal and forms an optical image by a change in a reflection angle. Mirror elements 21 corresponding respectively to pixels are formed in a matrix pattern, and each of the mirror elements 21 inclines ±θ° with respect to a plane 22 perpendicular to the optical axis of the projection lens by an ON signal for a white display and an OFF signal for a black display. After passing through a cover glass 23, an illuminating chief ray 24 enters and is reflected by the mirror element 21 and leaves the cover glass 23 again.
As shown in FIG. 11A, first, the incident angle of the illuminating chief ray 24 is set such that, at the time of the ON signal, an ON light chief ray 25 is reflected and travels along a direction perpendicular to the plane 22, that is, the optical axis of the projection lens. In this case, the illuminating chief ray 24 and the ON light chief ray 25 form an angle of 2θ. On the other hand, as shown in FIG. 11B, at the time of the OFF signal, an OFF light chief ray 26 is reflected and travels along a direction not reaching the projection lens, and the illuminating chief ray 24 and the OFF light chief ray 26 form an angle of 6θ.
As shown in FIG. 10, an illuminating light 8 entering the reflecting light valve 6 reaches the projection lens 7 as ON light 9 at the time of the ON signal or travels outside an effective diameter of the projection lens 7 as OFF light 10 at the time of the OFF signal. By controlling the time allocation of the ON light 9 and the OFF light 10 according to the video signal as described above, a projected image is formed on the screen.
However, reflected light generated at an interface of the cover glass 23 shown in FIG. 11 and the air as an external medium travels as unwanted reflection light 11, which is shown as hatching in FIG. 10, and partially enters the projection lens 7. Since this unwanted reflection light 11 travels similarly in both times of ON/OFF signals, it considerably reduces the quality of black display especially at the time of OFF traveling, causing a deterioration of contrast performance.